The present invention relates generally to remote reporting systems for line elements in a digital transmission network. More particularly, the present invention relates to a system for enabling a line element, such as a regenerative repeater, that is interconnected to a transmission line in a digital transmission network to efficiently communicate its address to a remote location that is also interconnected to a transmission line in the network, without affecting the status of other line elements. This invention may, for example, assist a telephone company technician in identifying, from a test location, the location of a particular repeater, in a string of repeaters, that is malfunctioning.
The present invention may be used with digital transmission line networks generally, including, for example, the Regional Bell Telephone Systems in the United States. A general description of such a network is provided in U.S. patent application Ser. No. 07/943,859, filed on Sep. 11, 1992 by Pesetski et al. ("Pesetski"), for a remote reporting system for digital transmission line elements.
As Pesetski notes, the data, or "payload," signals on digital transmission lines are typically sent differentially on a Tip-Ring pair. Payload signals are received by equipment at the telephone company central office and, generally, are transmitted, via cables, to a series of regenerative signal repeaters, and ultimately to customer premises equipment. Such repeaters are spaced along the cables approximately every 6,000 feet. Each length of approximately 6,000 feet of cable may be designated as a "span."
The first repeater receives the data from the central office, but, because of transmission line losses, noise, interference, and distortion, the signal will have degenerated. Therefore, the repeater recognizes the presence or absence of a pulse at a particular point in time and, thereafter, if appropriate, regenerates, or "builds up," a clean, new pulse. The first line repeater (or "signal repeater" or "regenerative repeater") sends the regenerated, or repeated, signal to the next line repeater, stationed approximately one mile away, and so forth, until the signal reaches its destination.
The Bell Telephone System has widely utilized time multiplexed pulse code modulation systems. Such systems have generally been designated as "T carriers." The first generation of multiplexers designed to feed the T1 system was the D1 channel bank. Channel banks have evolved through the D5 series. The "D" channel bank provides multiple DS-1 signals that are carried on the T1 systems. Each T1 system carries twenty-four two-way channels on two pairs of exchange grade cables. One pair of cables provides communication in each direction.
For convenience and simplification of terminology, the pair of cables carrying signals from the central office to the customer premises equipment may be referred to as a "transmit" line, and the pair of cables transmitting data from the customer premises equipment to the central office may be referred to as a "receive" line. These designations are made only as a matter of convenience; when an observer (such as a testing technician) changes position from a central office to the customer premises, what used to be a "transmit" line becomes a "receive" line, and what used to be a "receive" line becomes a "transmit" line.
As further indicated by Pesetski, the data to be transmitted over the cables, such as speech, may be sampled at a rate of 8,000 hertz, and the amplitude of each signal is measured. The amplitude of each sample is compared to a scale of discrete values and assigned a numeric value. Each discrete value is then encoded into a binary form. Representative binary pulses appear on the transmission lines. The binary form of each sample pulse consists of a combination of eight pulses, or bits. The eighth bit is robbed every sixth frame (or 750 microseconds) to allow for signaling.
A coding system is typically used to convert the analog signal to a digital signal. The system guarantees some desired properties of the signal, regardless of the pattern to be transmitted. The most prevalent code in the United States is bipolar coding with an all zero limitation (also called "AMI" for Alternative Mark Inversion). With bipolar coding, alternate "ones" are transmitted as alternating positive and negative pulses, assuring a direct current balance and avoiding base-line wander. Further, an average density of one pulse in eight slots, with a maximum of fifteen zeros between "ones," is required. This is readily obtained in voice-band coding, however, by simply not utilizing an all-zero word.
In many telecommunication systems, data may be transmitted sequentially in discrete groups of bits called "frames." In the T1 system, for instance, all twenty-four channels are sampled every 125 microseconds (equivalent to 1/8,000 of a second), constituting one frame. A synchronizing bit, or "frame bit", is added to each frame to serve as a flag, enabling line elements to distinguish each frame from the preceding frame or from noise on the line. Since there are eight bits per channel and there are twenty-four channels, and there is one frame bit at the end of each frame, the total number of "bits" needed per frame is 193. Thus, the resulting line bit rate for T1 systems is 1.544 million bits per second.
Signals that violate either the coding rules or the framing rules established in a particular system are detected as errors. Thus, for example, under a bipolar coding scheme, two positive pulses should never occur in sequence. To the extent such pulses do occur adjacent to each other (as detected, for instance, by a test set applied to the digital transmission cables), such a signal may be noted as a bipolar coding violation. Similarly, as described in U.S. patent application Ser. No. 08/152,724, filed on Nov. 15, 1993 by Bergstrom et al. ("Bergstrom"), a digital signal that violates framing rules (such as framing bit requirements) established in a given system is detected as "frame error." In a given encoding protocol, a sufficient number of frame errors may be detected as a frame loss.
In a telecommunications network, signals generally pass from customer premises equipment, through at least one central switching station, and on to other customer premises equipment. Thus, in a telecommunication network such as the Bell Telephone System, equipment at the central office may serve as a hub to a plurality of transmission line branches, each of which connects equipment at given customer premises to the central office. Each transmission line branch comprises a series of line elements (such as regenerative repeaters) interconnected to one another by spans of cables comprising both transmit and receive lines. Along a given branch, every line element is identified by a unique address, which may take the form of a 16-bit binary code, to distinguish each line element from the others. However, it is known that in some systems, a line element on a first branch connected to the central office may have the same address as a line element on a second branch connected to the central office.
As Pesetski notes, there may be many miles of cable between the central office and a given customer premises, with a large number of repeaters along the branch between the two facilities. Thus, if the malfunction of a transmission line is detected during a test (or simply during normal operation), it is important to make an accurate determination of the location of the fault. In this way, the fault may be located and corrected more quickly and inexpensively.
One method of determining the location of a fault along a digital transmission line branch is by selectively placing line elements along the branch in "logical loopback mode." The unique addresses of the line elements along each branch facilitate this selective "sectionalization," as discussed more fully below.
The use of loopback to determine the location of a fault along a digital transmission line is disclosed, for instance, in U.S. Pat. No. 5,224,149, by Garcia (assigned to the assignee of the present invention). As Garcia explains, an activating signal may be sent by the test set located, for instance, in the central office. This activating signal is frequently referred to as a "loopup" code, because it is intended to cause a line element to enter loopback mode. The activating signal may designate a first repeater to "loopback" the signal from the transmit line to the receive line. Accordingly, a signal sent down the transmit line should then be received immediately thereafter at the receive line in the central office, if the lines to and from the repeater are continuous and the repeater has performed a loopback between the transmit and receive lines.
Should continuity between the test set and the first repeater thereby be proven, the test set may then instruct the first repeater to connect the lines in standard transmission mode and instruct the next most proximate repeater to loopback signals. Thereafter, if the test signal applied to the transmit line is not then also received at the receive line, the telephone company technician will know that the malfunction has occurred between the loopback of the first repeater and the loopback of the second repeater. The error in the line will thereby be sectionalized to a 6,000 foot interval rather than the entire length of the transmission line.
Notably, logical loopback may occur toward either the central office or the customer premises equipment. As suggested above, when loopback occurs toward the customer premises equipment, the arbitrary designations of "transmit line" and "receive line" are reversed.
In summary, when a line element such as a repeater is placed in loopback mode, it receives data on the transmit line (from the direction of the test set) and loops the data back onto the receive line (toward the test set), rather than building-up and passing the data along the transmit line to the next line element. Typically, while in loopback, a line element will generate a "loopback indication signal" (or "LIS") along the (outgoing) transmit line, to indicate its loopback status. A loopback indication signal may generally comprise a continuous string of logical "1" bits.
In order for a test set to place a selected line element in loopback mode, it is necessary to establish communication between the test set and the line element. In some systems, a supplemental transmission line (in addition to the transmit and receive lines) is installed for carrying such control or "maintenance" signals from one location to another along the network transmission lines. In other systems, as disclosed by Pesetski and Bergstrom, for instance, maintenance signals may be carried within the continuous stream of data in the existing transmission lines in the form of intentionally generated bit (coding) errors or frame losses in the transmitted data.
As indicated in Bergstrom, however, it has been determined that intense noise in the form of "dribbling" bit errors along the transmission lines may, under extreme conditions, render communication, via bit errors, more difficult to accomplish. Further, communication through the transmission of frame losses necessarily requires the exacting synchronization and control of framing in the transmitted data.
In other systems, in order to provide maintenance communication between the test set and a selected line element, the test set first arms the line elements along the branch (by transmitting an arm signal) and then transmits a loopup code directed to the selected line element. The selected line element receives the loopup code and responds by first sending a confirmation signal to the test set and then entering loopback mode.
In one system, for instance, a test set connected to a line element or central office begins by transmitting along the transmit line five seconds of an "arm" code, to notify the line elements along the transmit line that one of those line elements may be requested to enter loopback. Next, the test set transmits three seconds of a loopup code, in order to command the selected line element to enter loopback mode.
Generally, the loopup code transmitted by the test set may comprise the address of the selected line element. However, in order to avoid unintended loopup, it is important for the selected line element to ensure that its address, rather than noise, is in fact being transmitted by the test set. Therefore, during the three seconds, a loopup code is transmitted repeatedly within a fixed bit order, and the selected line element continues to look for a matching code.
For example, assuming the use of 4-bit line element addresses, the test set may send the 4-bit address of the selected line element, repeated for three seconds. If the address of the selected line element is 1011, for instance, the repeatedly transmitted pattern would therefore appear as follows: EQU 1011101110111011 . . . 10111011 EQU time=5 seconds
In turn, a detector in the selected line element analyzes the transmitted data stream in search of the matching pattern, 1011.
Importantly, in a repeated pattern like that shown above, more than one 4-bit pattern is repeated, depending on the point in the data stream at which the detector begins analyzing the data. In the above example, for instance, the detector might not start looking at the incoming data until the second bit is received. In this scenario, the data stream seen by the detector would appear as follows: EQU 011101110111011 . . . 101110111 EQU time=5 seconds
To the detector, this pattern may therefore appear to be the 4-bit code 0111 repeated continuously. However, the detector must also recognize that this pattern is 1011 repeated continuously as well. In this respect, the 4-bit code 1011 is the same as the 4-bit code 0111 "rotated."
Where a binary word can be transformed into a different binary word simply by rotating one of the words, the two binary words are said not to be "unique under rotation" with respect to each other. Therefore, the 4-bit codes 1011 and 0111 are not unique under rotation with respect to each other. Conversely, the binary words 1011 and 1001, for instance, are unique under rotation with respect to each other, because, regardless of the number of times the word 1011 is rotated, the word 1001 will never result. Thus, for example, considering all possible 4-bit word combinations and their rotations, the following table indicates which codes are unique under rotation with respect to the others.
______________________________________ Equivalent (Decimal Unique Decimal Value) Under Code and N-1 Rotations Value Number Rotation ______________________________________ 0000 .fwdarw. 0000 .fwdarw. 0000 .fwdarw. 0000 0 0 * 0001 .fwdarw. 0010 .fwdarw. 0100 .fwdarw. 1000 1 1 * 0010 .fwdarw. 0100 .fwdarw. 1000 .fwdarw. 0001 2 1 0011 .fwdarw. 0110 .fwdarw. 1100 .fwdarw. 1001 3 3 * 0100 .fwdarw. 1000 .fwdarw. 0001 .fwdarw. 0010 4 1 0101 .fwdarw. 1010 .fwdarw. 0101 .fwdarw. 1010 5 5 * 0110 .fwdarw. 1100 .fwdarw. 1001 .fwdarw. 0011 6 3 0111 .fwdarw. 1110 .fwdarw. 1101 .fwdarw. 1011 7 7 * 1000 .fwdarw. 0001 .fwdarw. 0010 .fwdarw. 0100 8 1 1001 .fwdarw. 0011 .fwdarw. 0110 .fwdarw. 1100 9 3 1010 .fwdarw. 0101 .fwdarw. 1010 .fwdarw. 0101 10 5 1011 .fwdarw. 0111 .fwdarw. 1110 .fwdarw. 1101 11 7 1100 .fwdarw. 1001 .fwdarw. 0011 .fwdarw. 0110 12 3 1101 .fwdarw. 1011 .fwdarw. 0111 .fwdarw. 1110 13 7 1110 .fwdarw. 1101 .fwdarw. 1011 .fwdarw. 0111 14 7 1111 .fwdarw. 1111 .fwdarw. 1111 .fwdarw. 1111 15 15 * ______________________________________
In order to ensure that only the address of the selected line element is transmitted as a loopup code by the test set, the system must ensure that no rotated version of the selected line element corresponds to the address of another line element on the transmit line. Therefore, each of the line elements along a given transmit line must be defined by a binary address that is unique under rotation with respect to the addresses of the other line elements along the transmit line. In this way, of the line elements along a given transmit line, only the selected line element will detect the loopup code.
Once the selected line element assuredly detects its address in the incoming data stream (along the transmit line), the selected line element sends an acknowledgement signal (along the receive line) back to the test set to indicate that it is about to enter loopback. The acknowledgement signal may comprise, for instance, two seconds of the same address code just sent by the test set. After these two seconds of an acknowledgement code, the selected line element enters loopback mode, looping back data from the transmit line to the receive line.
To generally summarize, in order to detect the location of a fault downstream from a test position on a given transmission branch, a technician may connect a test set to the test position and configure the test set to cause a selected line element on the branch to enter loopback mode. To do so, in part, the test set may transmit along the transmit line the 16-bit address of the selected line element. The selected line element may in turn acknowledge that it will enter loopback mode, by transmitting its 16-bit address back to the test set.
Unfortunately, as noted above, it is possible that a line element on one branch connected to a central hub may bear the same 16-bit address as a line element on another branch connected to the hub. Thus, when the selected line element transmits its address (as an acknowledgement code) back to the test set, it is possible that the transmitted address may continue along the transmission lines past the test set, through the hub, and down another branch to the other line element that possesses the identical address. As an undesired result, if the other line element is armed, it may interpret the transmission of "its" address as a loopup code, and it may accordingly enter loopback as well.
Disadvantageously, when two line elements are simultaneously in loopback mode, communication difficulties may arise. As an example, a test set positioned at the central office may cause a first repeater on a first branch to loop data back to the test set. Simultaneously, the acknowledgement signal sent from the first repeater to the test set may pass to a second branch and cause a second repeater (on the second branch) to loop data back to the test set. Both repeaters in loopback would thus be sending loopback indication signals along their respective transmit lines to their respective customer premises equipment. Accordingly, both customer premises equipment at the end of the first branch and at the end of the second branch would simultaneously be receiving a loopback indication signal. In such a case, a test set at the end of either branch would be unable to signal either repeater to leave the loopback mode.
The presently available apparatus and methods for preventing such an undesired result involve the installation of blocking circuitry in the central office, the transmission of maintenance signals on a supplemental wire, or the transmission of maintenance signals in the form of bit errors or frame losses. However, as suggested in part above, these existing solutions suffer from one or more drawbacks. Installation of blocking circuitry or transmission on supplemental wires, for instance, may be both cumbersome and expensive. Transmission of maintenance signals in the form of frame losses may require the installation of synchronizing circuitry to ensure proper control of framing. Finally, transmission of maintenance signals in the form of bit errors may be difficult to accomplish where substantial noise (in the form of dribbling bit errors, for instance) exists along the transmission lines. A need therefore exists for a communication system in which a digital transmission line element can acknowledge its receipt of a loopup command without unintentionally causing other line elements to enter loopback as well.